1. Field of the Invention
The invention relates to cathode ray and other display devices suitable for use in high, low, and intermediate ambient light level conditions and, in particular, to a laminated combination of optical filter and electroluminescent graticule devices for operation in a wide range of ambient light levels.
2. Description of the Prior Art
Many airborne displays, as well as displays employed in ground-based air traffic control, radar, data processing, and the like systems have unusual requirements generally not fully met by conventional apparatus. A major need is to provide adequate brightness and especially good contrast when the display is viewed in high level ambient light, such as sunlight, while retaining these characteristics when viewed at very low light levels. A further major problem with such displays is connected with supplying suitably viewable fixed reference indices so that points of interest in the display can be readily located in relative position. Connected with the inherent natures of the displays themselves is the need for the lighting of the indices to be compatible over a large range of circumstances with the display brightness and with ambient light level conditions.
In aircraft cockpit instruments, several known attempts have been made to solve these problems, such as adjustable edge lighting of a transparent light guiding sheet placed in front of the display and bearing engraved markers which scatter light into the observer's eyes. This method and methods involving adjustable flood lighting of such indices fall short of acceptability, generally because they scatter considerable light unnecessarily into the cockpit, consume a substantial amount of power, and require too much space in already crowded aircraft instruments. Some attempts have been made specifically to place the needed indices on the inside surface of the cathode ray tube face plate at the location of the display phosphor. This arrangement very substantially reduces parallax between the electron beam generated scene and the index marks, but is considered to be expensive. Also, the concept lacks flexibility in that the index can not be modified once the display tube is manufactured. Further, such marks can not be readily viewed through a light-absorbing, contrast-enhancement filter applied to the external face of the display tube. Other methods, dependent upon illumination of indices by light scattered within the display, arising either from ambient light or electron-beam stimulated phosphor emission, are subject to the variability of the level, the distribution, and the angle of incidence of the light, and are not readily controlled. Accrodingly, it is an object of the invention to make the generation of light directed to the observer's eye by the display and that by the associated index device relatively independent of each other by means not characterized by the defects of the prior art.